1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus.
2. Description of the Related Art
In Japanese Patent Laid-Open No. 2000-77644, FIG. 1 illustrates a conventional photoelectric conversion apparatus using a phototransistor. As shown in this figure, the photoelectric conversion apparatus includes a common source circuit including a constant current source and a FIELD EFFECT TRANSISTOR driven by the constant current source. A voltage between a gate and a source of this MOSFET determines a base potential of the phototransistor. In the photoelectric conversion apparatus shown in this figure, when the amount of light incident on the base of the phototransistor changes, a corresponding change occurs in a collector current of the phototransistor, and a voltage between its base and emitter changes. This change of the voltage between the base and the emitter is mainly due to not a change in the base potential but a change in the emitter potential of the phototransistor.
However, there is a feedback loop between the emitter and the base of the phototransistor, and this feedback loop causes the base potential to have a small change. That is, when the emitter potential changes, the gate potential of the MOSFET connected to the emitter changes, and thus a change occurs in the voltage between the drain and source of the MOSFET forming the common source circuit. A constant current supplied from a constant current source flows through the MOSFET. To compensate for a change in current due to a change in drain-source voltage, the gate-source voltage changes although the change is slight. As a result, a change occurs in the base potential of the phototransistor. A PN junction including the base of the phototransistor functions as a photoelectric conversion element, and a reverse bias voltage applied to this photoelectric conversion element changes. This change in voltage occurs through charging of a parasitic capacitance associated with the base by a photocurrent. The time needed in this charging operation determines the optical response characteristic, i.e., the speed at which the current outputting from a terminal Iout changes in response to the change in the amount of light.
In view of the above, the present invention provides a photoelectric conversion apparatus having a suppressed change in a reverse bias voltage applied to a photoelectric conversion element due to a change in the amount of light and thus having a good optical response characteristic.